If you don't need POSIXt types, as Gabor says don't use them. However, there are good reasons to use them sometimes, and the most workable solution I have found is to set your default timezone in R to a non-DST timezone before you convert from character to POSIXct. This is dependent on your OS and particular build of R, but on Windows and Linux I have found that
Sys.setenv(TZ="Etc/GMT-2")
at the beginning of my R session should handle Central European Standard Time. To identify which time zones are supported on your system, read the documentation (there is generally a zoneinfo directory somewhere with filenames matching the time zones).
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B Laura <gm.spam2011@gmail.com> wrote:

Dear Gabor

http://rwiki.sciviews.org/doku.php?id=tips:data-io:ms_windows&s=excel
doesnt describe handling dates with daylight saving time issues.

R classes Date can remove time and timezone, however calculating days
difference between two manipulated variables same problem appear if handling
these without Dates.

R News 4/1 doesnt provide solution to this neither.

Have read and struggled with this stuff for 3 days.

Anyone else who could help on this?

Regards,Laura.


2011/7/12 Gabor Grothendieck <ggrothendieck@gmail.com>

> On Tue, Jul 12, 2011 at 6:58 AM, B Laura <gm.spam2011@gmail.com> wrote:
> > Hello all,
> >
> > Could someone help me with the time zones in understandable & practical
> way?
> > I got completely stucked with this.
> >
> > Have googled for a while and read the manuals, but without solutions...
> >
> >
> >
> >_____________________________________________

> >
> > When data imported from Excel 2007 into R (2.13)
> >
> > all time variables, depending on date (summer or winter) get (un-asked
> for
> > it!) a time zone addition CEST (for summer dates) or CET (for winter
> dates).
> >
>
> Read
> http://rwiki.sciviews.org/doku.php?id=tips:data-io:ms_windows&s=excel
> which gives many ways of reading Excel into R and read R News 4/1
> which discusses appropriate R classes to use (you would b best to use
> Date, not POSIXct, in which case you could not have time zone problems
> in the first place) and internal representations of R vs. Excel.
>
> --
> Statistics & Software Consulting
> GKX Group, GKX Associates Inc.
> tel: 1-877-GKX-GROUP
> email: ggrothendieck at gmail.com
>

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